Server Driven UI Workflow

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Server Driven UI workflow Benoît Quenaudon @oldergod Cash App

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Cash 1.0

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Steps

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Steps • LINK_CARD • IDENTIFICATION_VERIFICATION (IDV) • CONFIRMATION

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enum Steps { LINK_CARD = 1; IDV = 2; CONFIRMATION = 3; }

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Cash 2.0

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enum Steps { LINK_CARD = 1; IDV = 2; CONFIRMATION = 3; }

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Blockers

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message IdentityVerificationBlocker { optional bool requires_birthdate = 1; optional bool requires_full_ssn = 2; optional bool requires_last_four_ssn = 3; optional bool requires_full_name = 4; optional bool requires_address = 5; }

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...and the app kept growing

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Server sends back ExecuteContractResponse if (response is ExecuteContractResponse) { return R.layout.status_result_view; } Pass ExecuteContractResponse#statusResult to the view

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Server sends back ExecuteContractResponse if (response is ExecuteContractResponse) { return R.layout.status_result_view; } Pass ExecuteContractResponse#statusResult to the view

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Server sends back ExecuteContractResponse if (response is ExecuteContractResponse) { return R.layout.status_result_view; } Pass ExecuteContractResponse#statusResult to the view

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message StatusResult {a }d

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B C D message StatusResult {a enum Icon {b SUCCESS = 1; PENDING = 2; FAILURE = 3; ACTION_REQUIRED = 4; NONE = 5; }c optionaloIcon icon = 1; }d

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a C D message StatusResult {a enum Icon {b SUCCESS = 1; PENDING = 2; FAILURE = 3; ACTION_REQUIRED = 4; NONE = 5; }c optionaloIcon icon = 1; optionalostring text = 2; }d

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B a C message StatusResult {a enum Icon {b SUCCESS = 1; PENDING = 2; FAILURE = 3; ACTION_REQUIRED = 4; NONE = 5; }c optionaloIcon icon = 1; optionalostring text = 2; optionaloStatusResultButton primary_button = 3; optionaloStatusResultButton secondary_button = 4; }d message StatusResultButton { enumoButtonAction { HOME_SCREENz=z1; LINK_CARDz=z2; OPEN_URLz=z3; } optionalnButtonAction actionz=z1; optionalnstring textz=z2; optionalnstring urlz=z3; }

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B a D message StatusResult {a enum Icon {b SUCCESS = 1; PENDING = 2; FAILURE = 3; ACTION_REQUIRED = 4; NONE = 5; }c optionaloIcon icon = 1; optionalostring text = 2; optionaloStatusResultButton primary_button = 3; optionaloStatusResultButton secondary_button = 4; optionalostring promo_textw=w5; }d

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message StatusResult {a enum Icon {b SUCCESS = 1; PENDING = 2; FAILURE = 3; ACTION_REQUIRED = 4; NONE = 5; }c optionaloIcon icon = 1; optionalostring text = 2; optionaloStatusResultButton primary_button = 3; optionaloStatusResultButton secondary_button = 4; optionalostring promo_textw=w5; }d

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...and the app kept growing

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Cash 3.0

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• Skippability • Ordering • Scalability

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Form Blocker

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ButtonElement LocalImageElement RemoteImageElement MoneyElement SpacerElement TextElement CustomizedCardElement AddressElement TextInputElement OptionPickerElement DetailRowElement CurrencyConversionFlagsElement

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message FormBlocker { oneof Element { ButtonElement button_element = 1; LocalImageElement local_image_element = 2; RemoteImageElement remote_image_element = 3; MoneyElement money_element = 4; SpacerElement spacer_element = 5; TextElement text_element = 6; CustomizedCardElement customized_card_element = 7; AddressElement address_element = 9; TextInputElement text_input_element = 10; OptionPickerElement option_picker_element = 11; DetailRowElement detail_row_element = 12; CurrencyConversionFlagsElement currency_conversion_flags_element = 13; } repeated Element elements = 1; optional BlockerAction primary_action = 2; optional BlockerAction secondary_action = 3; } ButtonElement LocalImageElement RemoteImageElement MoneyElement SpacerElement TextElement CustomizedCardElement AddressElement TextInputElement OptionPickerElement DetailRowElement CurrencyConversionFlagsElement

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Option Picker Element Primary Action Text Element Primary Action Text Input Element Text Element

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Data Format Sharing

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[On iOS, we use a] convoluted series of script

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Challenges

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Backporting

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Percent of users active per app versions since January 2019

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0.744% ≈ 50･000 customers

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• Force update • Redirect to Web App • Smoke&Mirror usage of old blockers • Use fallback blocker • Don't send blocker - skip it

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• Backporting • Bug Hunting • Complexity • Integration Challenges

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Pros

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• Real logic lives in only one platform • Lot of features/ﬁxes don't involve clients at all • Client logic is simple - to some degree • Changes available without client updates